Some conventional wireless communication devices include a closed-loop transmit power control system for amplification and transmission of a wireless signal. Such systems include a power control feed-forward path and a transmit power detector feedback path. A power amplifier or voltage controlled amplifier (VCA) along the power control feed-forward path amplifies the signal to be transmitted. The transmit power detector feedback path detects the power of the amplified signal, and determines and provides control or bias voltages to apply to the system's power amplifier or VCA, thus controlling the transmit power ramp up and ramp down curves.
Because of the closed-loop design, loop delay and gain are inherent in such transmitters. In order to preserve closed-loop stability and to enhance performance, traditional closed-loop transmit power control systems attempt to compensate for loop delay and gain during operation. However, compensation for loop delay and gain is relatively complex, because loop delay and gain may vary significantly as a function of variables such as supply voltage (e.g., battery voltage or Vbatt), temperature, frequency band of operation, manufacturing process variations, and/or characteristics of the slope of the control voltage versus the transmit power output curve, among other things.
Some traditional closed-loop transmit power control systems include factory-programmed loop control parameters (e.g., loop delay and loop gain values). The manufacturing process for each one of such devices includes a time-consuming factory calibration procedure in order to generate the factory-programmed loop delay and gain values for each frequency band of operation, and for different values within a range of anticipated operating power levels, steps of power change, and variations in temperature, battery power, and frequency bands of operation. Although factory calibration may provide adequate system performance, the calibration process is time-intensive, complex, and costly.
Other traditional closed-loop transmit power control systems preserve control loop stability by estimating loop delay and gain. However, these systems utilize nominal initial gain and delay estimates, which depend on the input signal dynamic ranges. As with the systems that utilize factory-programmed loop delay and gain values, calibrations of gain and delay parameters have to be performed across different operational power levels in order to meet system stability. In addition, neither of the above-described types of traditional systems is adapted to dynamically adjust the loop delay and gain, which may be a desirable feature of a closed-loop transmit power control system. Accordingly, what are needed are methods and apparatus for automatically determining and dynamically adjusting loop delay and gain in a closed-loop transmit power control system, while decreasing manufacturing time and cost and meeting control loop stability and performance specifications.